Google said, "when rel="alternate" hreflang="x" is included in conjunction with rel="canonical" or 301s, not only will our indexing and linking properties be more accurate, but we can better serve users the URL of their preferred language."

If you have a multi-lingual site, you 100% want to check out the Google post.

JohnMu at Google was very excited about it, he wrote in a Google Webmaster Help thread saying Google has "a neat new technique that can be used on sites that use translated templates for content they make available for users worldwide."